1. Field of the Invention
The invention concerns a black-white liquid crystal display. Strictly speaking, such displays are bright-dark displays, since with such displays not only white light but also coloured light without any marked colour shift can be switched.
2. Description of the Prior Art
Black-white liquid crystal displays can be implemented by utilizing different physical effects. Most of these displays use a liquid crystal cell arranged between two crossed polarizers containing a nematic liquid crystal which is provided with a twist as a result of orientation layers on the cell plates and the addition of a chiral compound. The most usual display is the TN display. The twist angle .phi. here is 90.degree. and the product .delta. of cell thickness d and anisotropy of the refraction index .DELTA. n have values over 1 .mu.m. TN displays are low-cost and simple to make. They have good contrast but are not easily multiplexed.
Different display types have been developed to obtain enhanced multiplexability. The best-known of these have an STN double cell, SBE double cell, and the OMI displays. A simple STN cell has a twist of 240.degree. and a .delta. value of about 1 .mu.m. The pretilt .THETA. i.e. the position angle of the molecules in relation to the adjacent cell plate is only a very few degrees.
These cells can easily be multiplexed, have less angular dependence on the contrast than TN cells, but are coloured. To offset the colour effect, two STN cells with opposing twists are connected in series, forming a colour-neutralized STN display with a double cell. However, the colour neutrality exists only within a very narrow viewing angle range of about 10.degree..
A simple SBE cell is described in the paper by T. J. Scheffer and J. Nehring in Apple Phys. Let. 45 (10), 1984, S. 1021-1023 under the title "A new highly multiplexable liquid crystal display". This cell also has a pronounced colour--it actually appears yellow or blue. To obtain colour neutrality by using the supertwisted birefrigence effect, two cells with opposing twists are therefore connected in series, as is the case with STN cells. Each individual SBE cell has a 270.degree. twist, a&gt;5.degree. pretilt and a .delta. value of about 0.8 .mu.m. The multiplexability and the angular dependence on contrast and colour are still better than for STN double cells, but manufacture is considerably more difficult. In addition, this angular independence is still unsatisfactory in spite of the improved angular dependency of the colour effect. STN and SBE cells work with polarizers which are rotated by 30.degree. or 60.degree. angle to the orientation directions.
OMI displays have a very good white effect over a large angle range. However, the angular dependence of the contrast is unsatisfactory. OMI displays have cells with up to a 210 twist and .delta. values as low as 0,4 .mu.m, preferably 0.55 .mu.m. This sort of cell is described in two papers by M. Schadt and F. Leenhouts under the title "Electro-optical performance of a new black-white and highly multiplexable liquid crystal display" in Apple Phys. Let. 50 (5) 1987, pp 236-238 and under the title "The optical mode interference liquid crystal display: dependence on material and cell parameters" in SID 87 Digest. pp 372-375.
The advantage of these cells is that they can be easily produced and can have multiplex rates of Nmax=1000 lines. Maximum contrast is obtained with crossed polarizers. At least one of these is positioned parallel to an orientation direction. If the angle between the polarization directions is increased by over 90.degree. and if the polarizers are rotated in relation to the orientation directions, the brightness increases but the contrast decreases.